The largest study of its kind has found damage in the vast majority of former football players’ brains donated for research after they developed mental symptoms during life. Of 202 former players of the U.S. version of the game whose brains were examined, 87% showed the diagnostic signs of chronic traumatic encephalopathy (CTE), a neurodegenerative disease associated with repetitive head trauma. Among former National Football League (NFL) players in the sample, that number jumped to 99%. The findings will likely ratchet up the pressure on leaders at all levels of football to protect their players. Still, the authors and other experts caution against overinterpreting the results, because the brains all came from symptomatic former players and not from those who remained free of mental problems.

“I think it is increasingly difficult to deny a link between CTE and repeated traumatic brain injury, be it through contact sports or other mechanisms,” says Gil Rabinovici, a neurologist at the University of California, San Francisco (UCSF), who was not affiliated with the study.

The researchers, led by Boston University (BU) neuropathologist Ann McKee, used brains from a bank maintained by the VA Boston Healthcare System, BU, and the Concussion Legacy Foundation. They were donated by families of former football players. The team defines CTE, a diagnosis made only at autopsy, as “progressive degeneration associated with repetitive head trauma.” The designation remains controversial with some, who call it a muddy diagnosis that doesn’t include an iron-clad clinical course and the kind of clear-cut pathology that defines classical neurodegenerative diseases like Parkinson’s or Alzheimer’s disease.

The team first examined the 202 football players’ brains for CTE’s microscopic calling card: clumps of a neuronal protein called tau tagged with extra phosphate groups. Although such abnormal tau clusters are found in brain diseases including Alzheimer’s, their location in CTE is unique—near small blood vessels, and most often near the bottom of sulci, the deep folds in the brain’s cortex. The BU team defined mild CTE as consisting of tau clusters and sometimes another abnormality—twisted tau-containing fibers known as neurofibrillary tangles—in the sulci and other areas of the cortex. Severe CTE was defined as tau clusters and several other degenerative abnormalities affecting not only the cortex, but deeper brain structures including the hippocampus, the amygdala, and the brain stem.

The researchers found that, in addition to the NFL players, 48 of 53 brains of former college players showed signs of CTE. So did seven of eight who played professional football in Canada and nine of 14 semiprofessional players. The brains of college and professional players were more extensively affected: Eighty-six percent of former professional players had severe disease, and 56% of college players, the team reports today in The Journal of the American Medical Association. Former high school players with CTE—three were diagnosed out of 14—had mild brain disease as defined by the researchers. No CTE was found in the brains of two men who played football only before high school.

The researchers also completed extensive, standardized telephone interviews with family members of the dead men. The interviewers were unaware of the microscopic findings in any given donor’s brain. And the pathologists who examined the brains didn’t know about the symptoms and progression of the disease in any donor.

Photos of a normal brain (top) compared with the brain of Greg Ploetz (bottom), who played defensive tackle for the Texas Longhorns and who suffered from severe chronic traumatic encephalopathy.

Boston University Photography

The team found that—whether the men’s brain changes were mild or severe—all experienced mood, behavioral, or cognitive symptoms associated with CTE. These included impulsivity, depression, apathy, anxiety, explosive rages, episodic memory loss, and problems with attention and higher order thinking. Nearly all donors whose brains were diagnosed with CTE (96%) experienced progression of their symptoms during life. Men found to have milder brain pathology died younger, at a median age of 44, and suicide was their leading cause of death. The men with more severe pathology died at a median age of 71. Their leading cause of death was dementia or trouble with movements like swallowing.

But McKee and her co-authors stress that the rate of CTE in the study can’t be used to estimate the prevalence of CTE in all football players: The brains came from men who all had symptoms in life that moved their families to donate their brains for research. What’s more, they added, the sample is heavily weighted to men who played football in college or professionally, exposing them to far more hits to their heads than those who played only on youth or high school teams: These younger players accounted for only 16 brains in the sample of 202.

Questionnaire-based studies of living, retired NFL players have found rates of memory symptoms and depression between 5% and 20%—an imprecise comparison, but one that may reflect NFL players more broadly.

The current study was funded in part by NFL, through a $30 million donation the league made to the National Institutes of Health in 2012. The league said in a statement: “Case studies such as those compiled in this updated paper are important to further advancing the science and progress related to head trauma. The medical and scientific communities will benefit from this publication.” (NFL called the paper “updated” because 36 of the cases were first described in the journal Brain in 2013.)

Dirk Keene, a neuropathologist at the University of Washington in Seattle who is working with McKee on another project to quantify the pathological changes in CTE, notes that the fact that men with damaged brains developed symptoms does not prove that the brain pathology caused the symptoms. “Association is not the same as causation,” he says. It’s often overlooked, he adds, that “we don’t how a single CTE lesion, or several, or severe pathologic changes, translate to cognitive or behavioral impairment.” He also notes that the study’s results, because they are drawn from brains studied at a single point in time, can’t illuminate disease progression. For instance, it’s impossible to know whether the 13% of men whose brains showed no damage would have developed it with time.

The researchers also tested the brains for abnormal brain proteins indicative of other neurodegenerative diseases including Alzheimer’s and motor neuron diseases like amyotrophic lateral sclerosis. In 45% of the men, they found them. The suggestion is that repeated traumatic brain injuries may cause other brain diseases, too. But because the risk of these diseases increases with age, and because the team found that men with more severe CTE were more likely to have these accompanying diseases, the presence of the other diseases could be coincidental, and due to age.

Scientists in the CTE field say that much more research is now needed, including prospective studies to determine the incidence and prevalence of CTE both in football players and the wider population. These in turn depend on development of brain imaging tests or biomarkers that could be measured in the blood or other body fluids to allow a diagnosis of the disease in living people. Positron emission tomography scanning for tau is showing some promise, says UCSF’s Rabinovici. “But nothing is ready for prime time yet.”